Light-emitting diodes, short LEDs, are an important class of solid-stated devices that convert electric energy to light. LEDs are attractive candidates for replacing conventional light sources such as incandescent and fluorescent lamps. LEDs typically have substantially higher light conversion efficiencies than incandescent lamps, and have longer lifetimes than conventional light sources. Some types of LEDs have higher conversion efficiencies than fluorescent light sources.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of LED light sources. The conversion efficiency of an LED is defined to be the electrical power dissipated per unit of light that is emitted by the LED. Electrical power that is not converted into light is mainly converted into heat that raises the temperature of the LED. The light conversion efficiency of an LED decreases with increasing current through the LED.
LEDs are typically powered from a direct current, short DC, power source or a modulated square wave source so that a constant current flows through the LED while the LED is in the ON-state. The current value is set to provide high conversion efficiency. In the light sources with variable intensity, the intensity of the light is controlled by changing the duty factor of a modulated square wave so that the current flowing through the LED is at a value consistent with providing the desired efficiency. Such a modulated square wave form is hereinafter referred to as pulse-width-modulated signal, short PWM-signal.
Conventional light systems for use in buildings typically are powered from an alternating current, short “AC, power source”. Thus, a LED-based luminaire typically includes an AC/DC power conversion means. The cost of such an AC/DC power conversion means represents a significant fraction of the cost of a typically LED luminaire. In addition, the power losses in the power converter reduce the overall efficiency of the light source. In addition, such AC/DC converters are not as reliable as that of LEDs and hence, can limit the lifetime of the lighting system.
To avoid unnecessary costs and increase the power efficiency, LED luminaires have been proposed that operate directly from an AC power source without the power first being converted to DC. For example, LED luminaires that include two LED-strings of series-connected LEDs have been proposed. The one LED-string is powered on when the AC waveform is in the positive half of the sine wave, and the other LED-string is powered when the negative half of the sine wave. This simple driving scheme suffers from low efficiency and flickering. To improve the efficiency, luminaires that include a full wave rectifier have been proposed.
To accommodate the voltage of a typical building power source, hereinafter referred to as AC main voltage Vmains, a number of LEDs might be connected in a series to build an LED-string. A typical effective mains voltage in Europe is 230 Volts or in USA 110 Volts. In case the US mains voltage Vmains is applied, the respective peak voltage is 156 Volts. Thus, approximately 43 LEDs are needed that must be placed in series to withstand this peak voltage. However, the LED-string will cease to make light when the voltage drops to 118 Volts. As a result, light is only generated in approximately 30 Percent of the time.
In prior art document U.S. Pat. No. 7,936,135 B2 a reconfigurable LED array and lighting system is described, in which each LED in a series string is connected in parallel with a switch that shorts that LED, when the AC voltage across the string is insufficient to drive all of the LEDs in the LED string. By removing LEDs from the LED string, when the AC voltage is below the voltage needed to drive all of the LEDs, the duty cycle is substantially increased. However, the resulting light intensity varies approximately sinusoidal. In addition, the light source will still cease to make light when the AC voltage falls below a forward voltage Vf. This leads to an average number of LEDs that generates light over the cycle of the AC mains voltage which is still substantially less than one hundred Percent. Finally, the costs of the luminaire are increased by the number of switches needed to implement such a scheme.
In nowadays lighting systems a lot of additional functionality is needed. For instance, an active sensing element, such as a motion sensor or an corridor function or an actor, such as a door opener, or an air-fan are installed in the lighting systems of a building to increase the functionality of the lighting system. Additionally, control systems in a lighting system, such as multi-point control units or dimming arrangements or communication units are applied in the building systems to increase its functionality. Those additional functions need a power supply.
Thus, it is an objective to provide a direct AC driving circuit for an LED that avoids the above-identified drawbacks. Especially, the energy efficiency of an LED luminaire should be increased and the costs should be reduced. The provision of a low voltage supply for increasing the functionality of the lighting system should be obtained without decreasing the energy efficiency or increasing the costs of the lighting system.